Corrosion resistance of iron and steel material can be enhanced by adding an element such as chrome, molybdenum, tungsten and nitrogen, and various kinds of stainless steel materials and chrome molybdenum steel materials are utilized as a structure material existing in a corrosion environment. However, there is a problem that also in such high corrosion resistant iron and steel material, corrosion resistance is largely deteriorated at a welded portion. This is because that added elements such as chrome locally and unevenly exist in the welded portion, and corrosion occurs at a location where the elements are locally starved.
Friction stir welding (FSW) is a solid phase bonding method which makes a to-be bonded material plastically flow by stirring a location of the material heated by friction heat by a tool, thereby bonding the material. As compared with melt welding, treatment temperature of the friction stir welding is low, and heat influence on the bonding material is relatively small. Hence, the friction stir welding attracts attention as a dominant bonding technique for solving the problem of the deterioration of corrosion resistance in the bonded portion of iron and steel material having a large problem in stainless steel. For example, PTL 1 discloses a friction stir welding method of stainless steel material. PTL 2 discloses a high corrosion resistance surface treatment method to which the friction stir welding is applied.